1. Field of the Invention
The present invention relates generally to mass spectrometry, and more specifically to the use of ion traps for multistage (MS/MS) mass spectrometry.
2. Description of the Related Art
One of the strengths of ion traps is their ability to be used for multiple stages of mass analysis, which is commonly referred to as MS/MS or MSn. MS/MS typically involves fragmentation of an ion or ions of interest in order to obtain detailed information regarding the ion's structure. When performing MS/MS in an ion trap, there are various ways to activate ions in order to get them to fragment. The most efficient and widely used method involves a resonance excitation process. This method utilizes an auxiliary alternating current voltage (AC) to be applied to the ion trap in addition to the main trapping voltage. This auxiliary voltage typically has a relatively low amplitude (on the order of 1 Volt (V)) and a duration on the order of tens of milliseconds. The frequency of this auxiliary voltage is chosen to match an ion's frequency of motion, which in turn is determined by the main trapping field amplitude and the ion's mass-to-charge ratio (m/z).
As a consequence of the ion's motion being in resonance with the applied voltage, the ion takes up energy from this voltage, and its amplitude of motion grows. In an ideal quadrupole field, the ion's amplitude will grow linearly with time if the resonance voltage is continuously applied. The ion's kinetic energy increases with the square of the ion's amplitude and therefore any collisions which occur with neutral gas molecules (or other ions) become increasingly energetic. At some point during this process, the collisions which occur deposit enough energy into the molecular bonds of the ion in order to cause those bonds to break, and the ion to fragment. If sufficient energy is not deposited into the molecular bonds while the ion's amplitude grows, the ion will simply hit the walls of the trap and be neutralized, or the ion will leave the trap through one of its apertures. Efficient MS/MS requires that this loss mechanism be minimized. Consequently, the parameters which affect the rate at which the ion's amplitude grows, and the energy of the collisions which occur, are important in determining the overall efficiency of fragmentation.
One of the most important parameters which influences both processes is the frequency at which this resonance process takes place. This frequency is dependant on the Mathieu stability parameter Q, whose value is proportional to the amplitude of the main RF trapping voltage and inversely proportional to the m/z of the ion of interest. The operational theory of quadrupole fields determines that any ions that have a Q value above 0.908 have unstable trajectories in the ion trap and are lost (either by ejection from the trap or by impinging on a surface.) Consequently, at any given RF amplitude, there is a value of m/z below which ions are not trapped. This value of m/z is called the low mass cut-off (LMCO). Proper selection of the RF trapping voltage amplitude to be applied during the activation process therefore involves consideration of two important parameters that depend on the RF trapping voltage amplitude: first, the frequency of the ion's motion, which in turn determines the kinetic energy of the collisions, and; second, the LMCO.
Due to requiring some minimum ion frequency for fragmentation, Q values of approximately 0.2 or greater are normally required to obtain acceptable fragmentation efficiencies of the parent ions. Operation at higher Q values produces more energetic collisions and therefore can produce more efficient fragmentation of the parent ion; however, raising the Q also raises the LMCO, preventing more of the lower mass fragments to be observed. Thus, a compromise Q value must be chosen which is sufficiently high to allow efficient fragmentation, but minimizes the LMCO. For example, commercially available ion trap systems set a default Q value of 0.25. Operation at Q=0.25 means that the lowest mass fragment ion observable is 28% of the parent ion m/z ((0.25/0.908)*100=28%). While the value of Q can be reduced to decrease the LMCO and allow detection of lower-mass fragments (which may be desirable, for example, in applications involving identification of peptide or protein structures), the decrease in Q comes at the possible expense of decreased fragmentation efficiencies. Similarly, the value of Q may be increased from the default value to produce more energetic collisions (which may be required, for example, to fragment large, singly-charged ions), but such an increase in the Q value will have the undesirable effect of raising the LMCO precluding the detection of lower-mass fragments.
In view of the foregoing discussion, there is a need for an ion fragmentation technique for ion traps that avoids the tradeoff between fragmentation energies and LMCO inherent in the prior art resonance excitation process. There is a further need in the art for a ion fragmentation technique which produces fragmentation in a shorter period of time relative to the prior art process.